1. Field of the Invention
The present invention relates to a current sensor which measures a current passed through a primary conductor.
2. Description of the Related Art
When a high current is measured, because the direct passage of the high current through an ammeter is dangerous, a current sensor is used to measure the high current. In the current sensor, a primary current is decreased and supplied to a secondary side with a current transformer (CT), thereby measuring a secondary current. The current transformer is formed by utilizing an alternating-current characteristic, and the current transformer has a structure in which the secondary current is taken out from a coil formed by winding a conductor around a core (iron core).
However, even if the high current is measured with the current sensor, sometimes the current is not correctly measured because a magnetic flux generated in the core is saturated. Therefore, in order to suppress the magnetic saturation in the core to enable the correct current measurement, there has been proposed a method of decreasing magnetic flux density in the core by enlarging a cross-sectional area of the core or performing feedback of a CT output (secondary winding) to a tertiary winding (for example, see Japanese Patent Laid-Open Publication No. 2004-153222).
Such as an electric reactor, in a case that evenness of the magnetic flux density in the core becomes insignificant, in order to decrease the magnetic flux in the core, there has been proposed a method adopting a structure in which the core is divided and the divided cores are coupled with a gap inserted therebetween (for example, see Japanese Patent Laid-Open Publication No. 2004-95935). There has also been proposed a method of reducing the saturation of the relative permeability of the magnetic core by utilizing a mixture of magnetic material powders and non-magnetic material powders as the magnetic core (for example, see Japanese Patent Laid-Open Publication No. 2006-024844).
Conventionally, when the high current is measured with the conventional current sensor, it is necessary that the feedback circuit is separately provided in the current sensor in order to suppress the magnetic flux saturation in the core, which causes a problem in that the apparatus configuration is enlarged. Although the method of enlarging the cross-sectional area of the core can be also adopted for suppressing the magnetic flux saturation in the core, because of the enlarged size of the core, the small current sensor is hardly formed.
In the method of dividing the core as the countermeasure for suppressing the magnetic flux saturation, a leakage flux is generated because magnetic resistance is remarkably increased at the gap portion between the divided cores, which causes the decreasing the evenness of the magnetic flux density in the whole circumference of the core. Therefore, unfortunately the measurement accuracy of the current sensor is lowered.
In the method disclosed in Japanese Patent Laid-Open Publication No. 2006-024844, in which the saturation of the relative permeability of the magnetic core is relaxed by utilizing the mixture of magnetic material powders and non-magnetic material powders as the magnetic core, since the relative permeability is changed for a magnetic field, unfortunately sensitivity is changed when the magnetic core of the mixture is adopted for the current sensor.